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Facilitator QuestionsIt has been shown by Nicklas that stable connections between kinetochores and spindle microtubules are formed only when this junction is under tension. What properties could to attribute to the kinetochore-MT junction so it would become stable only when under tension? Recent evidence shows that chromosomes can initiate MT polymerization in association with either their arms or their kinetochores. What impact do you expect these features of at least some spindles to have on spindle structure and/or function? The flux of spindle MTs suggests that these polymers are polymerizing and depolymerizing at different places. Where are these events occurring, and how would you test your answer experimentally? What spindle components do you think are responsible for MT flux? How would you test your ideas? What do you suppose would happen if a single kinetochore became attached to two poles? How might the cell deal with such a problem?
The speed of chromosome motion is generally quite slow (micrometers per minute). Calculate the amount of force necessary to overcome the viscous drag on a chromosome at such speeds. Each motor enzyme can generate around 5 x 10-12 N, so how many motors of this kind would be required to move a chromosome? Does the answer surprise you in any way? What do you think your result suggests about force generation in mitosis?
This result poses several questions to which the answers now available are only speculation, but they are intriguing: why does a cell move its chromosomes so slowly? How is chromosome speed regulated so the motors that are present do not cause the chromosomes to speed up? Are the forces generated by the spindle being used for a more subtle purpose, such as tearing the spindle apart as the chromosomes move? |
